Set DHCP relay sourceInterface from LoadBalancerInterface config

[rajeshvenkata]

Issue #, if available:
https://github.com/aws/eks-anywhere-internal/issues/3731
Description of changes:
Set DHCP relay sourceInterface from LoadBalancerInterface config.

When LoadBalancerInterface is configured in TinkerbellDatacenterConfig, propagate the same interface to the DHCP relay's sourceInterface. This ensures the relay listens for DHCP broadcasts on the correct network interface where bare metal machines are located.

Previously only kube-vip used this interface setting, leaving the relay to auto-detect which could pick the wrong interface in multi-NIC setups.

Testing (if applicable):

Specified a dummy interface in loadbalancerinterface in TinkerbellDatacenterConfig and tried to create a cluster and verified that relay has the specified interface in the args

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.17%. Comparing base (a6f7473) to head (1078c7c).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #10421   +/-   ##
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  Coverage   70.17%   70.17%           
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  Files         670      670           
  Lines       41643    41644    +1     
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+ Hits        29221    29222    +1     
  Misses      10620    10620           
  Partials     1802     1802           

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